Magnetron-type phonograph pickup



Jun'ef21, 1949. H, K, RlCHARDs Y 2,473,820

MAGNETRON- TYPE PHONOGRAPH PICKUP Filed Feb. 20, 1947 A 2 Sheets-Sheet l i f\ i u g:

if? JNVENToR.

June 2l, 1949. H. K. RICHARDS K MAGNETRON-TYPE PHONOGRAPH PICKUP 2 Sheets-Sheet 2 Filed Feb. 20, 1947 1N VEN TOR.

Patentedv June 21, 1949 MAGN ETRON -TYPE PHONOGRAPH PICKUP l Hans Kohn Richards, Chicago, Ill., assignor, by mesne assignments, toRaytheon Manufacturing Company, a corporation of Delaware Application February 20, 1947, Serial No. 729,820

The present invention relates to a device or apparatus for converting mechanical motion into electrical energy. Specifically, the present invention is an improvement on the arrangements disclosed and claimed in an application for Letters Patent of the United States, Serial No. 690,992, led jointly on behalf of Albert E. Shaw and Robert J'. Moon on August 16, 1946.

In such devices as microphones and pick-up units for phonographs, the main problem has been to efficiently `and accurately convert mechanical motion into electrical energy or at least to a variable potential. This has been accomplished in many different ways. phonographs, for example, various devices such as crystal pickups and magnetic pickups have been extensively employed in recent years. Crystal pickups have various disadvantages in that some require special protection against moisture, the crystal being highly hygroscopic. Also, some crystals such -as those made of potassium sodium tartrate (Rochelle salt) become very erratic in operation at high temperatures, for example, temperatures over 110 degrees F. Crystal pickups are also very fragile and consequently are easily damaged in shipment. It would `be desirable to provide an electronic pickup which would require no special protection against moisture, which would be independent of the temperature of `the ambient air and which was sturdy and stable in adjustment so that it would not be damaged by shipment or the like. vMagnetic pickups which have Ibeen used to some extent do not have some of the disadvantages of the crystal pickups but, on the other hand, they are of relatively heavy `weight which is often unsatisfactory from the standpoint of use as the pick-up unit of a phonograph. It would be desirable to provide apparatus of the electronic vtype for converting mechanical motion to electrical energy, which is very light so as to eliminate the inherent disadvantage 'of the magnetic type pickup.

Accordingly, it is an Aobject of the present invention to yprovide a new and improved electronic means for converting mechanical motion to electrical energy having none -of the disadvantages enumerated above.

It is another-object of the 'present invention to provide a new and improved electronic pickup for phonographs.

Sti-l1 another object Aol? 4the present invention is to provide a pick-up unit for translating the vundullations of Iolf-recorded transcription of music er speech linto 'electrical impulses, which `pickup employs a simple diode and a magnetic `structure 5 Claims. (Cl. 179-100.41)

In connection with "1"5 wherebyy the diode may be operated as a. magnetron.

It is a further object of the present invention to provide a new and improved electronic pickup for phonographs which is of sturdy and foolproof construction, of low manufacturing cost and which kprovides very satisfactory operation in converting mechanical motion to variable frequency electrical energy over ka Wide range of frequency withoutdistortion or other undesirable characteristics.

t is another object ofthe present-l invention to provide a new and improved electronic pickup for phoncgraphs in which the moving parts are reduced to an absolute minimum whereby a very compact mechanism of small dimensions is provided having a minimum weight.

It is a feature of the present invention to provide an electronic pickupfor phonographs in which a miniature diode is provided with means for producing a .magnetic eld in the direction of the axis of the anode to operate the diode as a magnetron. In addition, a small, movable rod, wire or other mass of magnetic or high permeability material is .positioned outside the envelope of the diode s0 as to .control the stray flux, thereby varying the flux encompassed by the plate of the diode and consequently varying the plate current of the diode. The movable rod or wire .is adapted to Lbe connected by a very light mechanism with the needle or stylus of a phonograph whereby the mechanical motion of the stylus Ais readily converted to variations of electric current, namely, the plate current of the diode.

Further objects and advantages of the present invention will become 'apparent as the following 'description proceeds and the `features of novelty which lcliaracterize this 'invention will vbe pointed out with particularity in the claims annexed to and forming a 'part'of 'this specication.

For a better understanding of the present invention, reference may 'be `had to the accompanying drawings in which:

Figs. 1 'and '2 are curve diagrams 4to aid in understanding the theory vunderlying the present invention;

Fig. '3 is a schematic .diagram of the electric circuit employed `in 4the electronic pickup .of y,the present invention;

Fig. 4 `is a top or plan View of a portion of a uphonograph 'in which 'the tone arm vincludes an 'electronic 'pickup embodying the present invention;

Fig. 5 is an enlargedvsectional view taken .on Vlmet-J5 ofFig. 4;

Fig. 6 is a sectional View taken on line 6-6 of Fig.

Fig. 7 is a sectional view taken on line 1 1 of Fig. 5 assuming Fig. 5 shows a complete device; and

Figs. 8 and 9 are schematic diagrams taken approximately on line 8-8 of Fig. 7 to aid in understanding the operation of the electronic pickup of the present invention.

The principles governing the inuence of a magnetic field upon the path of an electron are now well known. For example, it is known that an electron traveling in a, uniform magnetic eld directed perpendicularly to the direction of motion of the electron will be caused to travel in a circular path. Such behavior occurs in the magnetron, for example, which is a vacuum tube in which the flow of electrons from the cathode to the plate is controlled by means of a magnetic field. For any particular value of plate voltage which is the voltage between the plate and cathode of such an electron discharge valve or vacuum tube, there is a cut-olf value of the magnetic flux density for the current through the tube at which cut-off value all electrons leaving the cathode are prevented from reaching the plate. In the magnetron the plate current may be Varied or cut off completely by changing either the electrostatic field between the plate and cathode or the magnetic field. The relationship for theoretical cut olf, presupposing that a homogeneous electric field exists throughout the area encompassed by the plate of the magnetron and that a circular plate magnetron is employed, may be expressed by the following formula:

=6fm/E., (l)

where H is the magnetic ux density or magnetic field in oersteds; Ep is the plate voltage; and T is the radius of the plate of the magnetron in centimeters. If the anode is not of circular shape, the formula differs only by a constant factor. In actual practice, cut o is not sharp but gradual due to numerous factors.

From the above equation, it is apparent that for a particular magnetron the radius of the plate would be constant so that only two variable parameters would be involved for controlling the plate current, namely, a variation of the magnetic field I-I or a variation of the plate Voltage Ep. Generally the plate voltage would be obtained from a constant voltage source so that the plate current may be varied from maximum to zero by varying the magnetic flux density.

For various values of plate voltage, a family of curves designated in Fig. l as Epi, Epz, Eps, Em and Eps can be obtained experimentally by connecting the magnetron in an electric circuit and for given values of plate voltage varying the magnetic field and measuring the plate currents. These curves are plotted with the abscissas representing the magnetic field or magnetic flux density H and the ordinates representing the plate current Ip. An examination of the family of curves of Fig. 1 indicates that between the dotted llines A and B each of these curves is substantially designated by the same reference numerals. The

curves of Fig. 2 represent the situation where the plate voltage of the magnetron equals the battery rvoltage applied thereto or, in other words, when there is no load impedance. An examination of these curves, which are the static characteristics of the magnetron, indicates that by increasing the magnetic eld H, the plate current Ip decreases and the slope of the curve, namely may be called the magnetic transconductance represented by the symbol 9H. Without the load impedance, the plate current may be expressed as follows:

AIp=gHAH (2) where gn is amperes per oersted and Alp and AH are increments of plate current and eld respectively.

In a practical application, it is desirable to connect an impedance in series with the plate circuit of the magnetron across which impedance variations in potential can be obtained which may be applied to suitable amplifying means or the like. In Fig. 3 there is illustrated in schematic form the circuit diagram of a magnetron in which the means for producing the magnetic eld has been omitted and a simple diode I0 having a cylindrical plate I I and a cathode I2 is illustrated. The cathode I2 forms part of a filament I3 which is connected across a suitable source of potential I4 for supplying the filament or cathode heater current. The plate circuit of the diode I0, which, when provided with means to produce a magnetic eld, effectively is a magnetron, includes a source of potential illustrated as a battery I5 connected in series with a suitable impedance I6 which is illustrated as a resistance but which may equally well comprise an inductance or other suitable impedance means. Connected across the impedance I6 through a coupling condenser IB is an amplifier generally designated at II which may comprise any suitable number of stages and in the case of an electronic pick-up unit would be connected to supply a suitable loud speaker or the like, not shown.

It will be understood that when the impedance I6 is connected in the plate circuit of the diode Il), the plate voltage will no longer be equal to the battery voltage but will be less by the voltage drop in the impedance I6. With an increase in the eld H the plate current Ip will decrease thereby decreasing the IZ drop in the impedance I5 and increasing the plate voltage Ep. For a particular impedance I6 and for a particular battery voltage for the battery I5, a curve such as F in Fig. 2 may be plotted which may be referred to as the dynamic characteristic of the magnetron and which has a slope of the same sign as the slope of the static characteristics but substantially less than the slope of the static characteristics. A family of these curves can be plotted for various plate voltages which would be parallel to the curve F, the slope of the curves being determined by the load impedance. rIhese dynamic characteristics such as F can be directly employed in connection with a magnetron since the variation in the magnetic field applied to the magnetron can be converted directly to Variations in plate current flowing through an impedance I6 across which a suitable impedance drop may be obtained. From an examination of the dynamic characteristic F of Fig. 2 it may be observed that, by increasing the magnetic field, the plate current Ip will also decrease, but not as rapidly as when no load impedance such as I6 is employed.

envelope through a suitable press seal 3|. spacing between the cathode and anode will del pend upon the characteristics which the electron discharge valve is `intended to possess. ample, the anode or plate II may'have a radius of the order of 0.25 centimeter, while the cathode I may have a diameter of the order of 0.0015 .l centimeter.

s The plate current of the magnetron may be expressed by the following equation in terms of 4the :tield H:

Similarly, the change in plate potential may be expressed by gHZ-AH .,-rgz 4) Where g is the transconductance in mhos and Z is the load impedance in ohms. This fundamental equation indicates that if a variation in H can be produced by mechanical motion, a con- `version to a. variable electrical current Ip ilowing through a load impedance is obtained. The arrangement described hereinafter is based upon `Equations 3 and 4 above. In view of the fact that the coupling condenser I3 only will pass changes of current or voltage, thechanges of eld, current and voltage only are important and consequently the above equations are expressed in increments of these quantities.

Referring now to Fig. 4 of the drawings, there is illustrated a suitable disc type record 20 which is mounted for rotation on a conventional turntable, not shown. A tone arm 2l pivotally lmounted as indicated at 22 is provided at its free end with a suitable pick-up unit of the electronic .type embodying the present invention generally ydesignated as 23.

As illustrated in the drawings, the tone arm 2i is formed of a suitable U-shaped `housing 'with the underside thereof open and the pick-up unit 23 is suitably mounted therein as by means of screws 2li threaded in suitable members 25 integrally formed with the tone arm 2l.

Referring specifically to Figs. 5, 6 and 7 of the ldrawings, the electronic pick-up unit 23 of the present invention essentially `comprises the diode -I schematically illustrated in Fig. 3 of the drawings, which diode includes a glass envelope 21 venclosing the cylindrical, annular or any other suitable plate or anode II, the cathode l2 and the filament or heater I3. The cathode is actually disclosed as of the directly heated type although it will be understood that an indirectly heated cathode may be employed. The cathode I2 comprises an elongated conducting member positioned substantially along the axis of the eny,velope 2.

non-magnetic material of which an alloy of Preferably vthe cathode is formed of chromium and nickel is Ia suitable type. Suitable The nected to the plate Il may suitably support the Y same and also electrically connect it with an eX- --ternal circuit. The envelope 2l is evacuated so as to provide an evacuated vessel, and the leads or conductors 28, 29Vand 30 enter the evacuated The For ex- For the purpose of providing a magnetic eld which is homogeneous throughout the `,volume confined within the :annular or cylindrical anode or plate I I, there is preferably `provided a permanent bar magnet generally indicated at 33 preferably formed of magnetic material of high permeability such as an alloy of aluminum, nickel and iron. It will be understood that a homogeneous magnetic eld can be produced by employing two bar magnets, such as the single bar magnet 33, one disposed at either side of the envelope 2l. In accordance with the present invention, however, the requirement of a second bar magnet to producea uniform or homogeneous magnetic field has been dispensed with and in its place there have been provided a pair of flux guides 34' and 35 which are in the nature of U-shaped members, one disposed at either end of the bar magnet 33 and suitably fastened thereto as by welding or the like indicated at 36. These iiux guides also perform a very desirable function in embracing the envelope 2 of the diode I3 as is apparent from Fig. 5 of the drawings.

As is best shown in Figs. 6 and 'l of the drawings, the pole shoes or flux guides 34 and 35 are positioned so that the adjacent ends thereof are substantially positioned adjacent the respective ends of the plate or anode II. The flux paths which, as will be understood by those skilled in the art, will follow the paths of minimum reluctance of the magnetic circuit will consequently extend through the 'bar magnet 33 and the pole sho-es or flux guides 34 or 35. Between the pole shoes 3123 and 35, vthe iiux will pass through the non-magnetic circuit including the evacuated envelope 2l, and a substantially homogeneous ilux distribution parallel with the axis of the envelope or vessel 2l willoccur within the volume encompassed by the plate or anode II preferably of cylindrical configuration. By reference to the schematic disclosure of Fig. 8 of the drawings, the distribution of iiuX between the pole shoes or flux guides 34 and 35 within the plate i I is illustrated. It will be understood that stray iiux will also exist around the outside of the plate il and even outside the glass envelope 2'! of the diode i0. However, the fiux within the cylindrical plate II is fairly uniform and homogeneous.

For the purpose of varying the magnetic field within the plate II of the diode I0 which effectively is a magnetron when provided with the magnet 33 and the flux guides 34 and 35, there is provided a mechanism by means of which mechanical motion can be caused to vary the eld of the magnetron. This means in accordance with the present invention comprises a suitable stylus or needle 38y mounted in a vibratory member 35 which is in turn mounted so as to extend through a suitable opening in a support 40 and held there by means of a set screw 4I. The support dil is mounted for oscillation by means of resilient members 42 formed of rubber or other similar material clamped in a suitable clamping mechanism 43 which is supported by the screws y2li from the extensions 23. The clamping mechthat mechanical motion of the stylus 38 will cause motion of the member 39 land oscillation of the support 43. By employing the resilient Aoscillation permitting means or members 42, such fmounting means will permit damped oscillations of the member 39. Inasmuch as the intended range of frequency of operation of the stylus 38 may be from frequencies below 50 cycles to as high as 6,000 or '7,000 cycles per second, the resilient members 42 are preferably formed of rubber or similarly resilient cushion material, the elas- `ticity of which permits rapid oscillations and pendicular to the axis of the support 40, the

member 39 is not cut away in this direction of force transmittal whereby sufficient strength or Vrigidity is retained as far as the transmission of desirable forces or vibrations are concerned while :at the same time reducing the movable mass to a minimum.

In accordance with the present invention, there is connected to the upper end of the member 39, as by welding or the like, a rod, wire or piece 44 of magnetic material. This member or rod 44 may comprise a piece of piano wire or the like and is arranged so as to be parallel with the axis of the evacuated envelope 21 and positioned closely adjacent thereto so as to extend between the flux guides 34 and 35, as is clearly shown in Figs. and 6. Vibrations or mechanical movements supplied to the stylus or needle 38 will cause the rod 44 to move toward and away from the glass envelope 2, the extreme positions thereof being shown schematically in Figs. 8 and 9 of the drawings. Figs. 8 and 9 also indicate the effect on the fiux distribution of the movement of the rod or wire 44. When the rod or wire 44 is, in the position indicated in Fig. 8, relatively remote from the plate Ii of the diode ICI, it is sufficiently removed from the stray flux so as to have a minimum effect in displacing the flux within the surrounding plate. Consequently, a maximum eld exists within the plate and by virtue of Equation 3 above the plate current is a minimum. As the wire or rod 44 moves into close proximity with the exterior of the glass envelope 2l', the member 44 tends to pull the stray iiux which surrounds it so as to pass through it since it provides a path of low reluctance for such flux. Consequently, the magnetic flux density within the plate II is reduced which is indicated in Fig. 9 by the reduced number of dotted lines. By virtue of the curve F of Fig. 2 of the drawings, or by virtue of Equation 3, it will be seen that with the minimum distance of the wire 44 from the plate I I the field becomes a minimum within the plate and the plate current becomes a maximum. It has been found that, with a `magnetic eld of between 50 and 350 gausses, with a yplate voltage oi between 8 and 50 volts, with 'the radius of the plate of the order of 1/4 of a centimeter, and with an impedance I6 of Fig. 3 of the drawings in the form of a resistance having a value of between 50,000 and 200,000 ohms, a range of voltage variation across the resistor I6 of le volt to one volt or more may be obtained which may readily be amplified to produce a high fidelity output.

In view of the detailed discussion included l above, the operation of the electronic pickup of the present invention when embodied in a circuit as illustrated in Fig. 3 of the drawings will be apparent. With the needle or stylus 38 following the grooves in the record 20 the undulations recorded in the groove will cause vibration or movement of the rod 44 of magnetic material so as to move toward and away from the axis of the glass envelope 21. Such movement of the wire 44 acting on the stray field will cause a variation of the magnetic flux density within the volume encompassed by the cylindrical plate II and in accordance with the theory expounded above, and the equations and curves will produce a variation in the plate current through a suitable load impedance I6 across which voltage variations may be obtained to be supplied to a suitable amplifier and loud speaker.

Although the structure of the present invention has been specincally illustrated as applied to the pick-up unit of a phonograph for translating the undulations of a transcription record into electrical energy, it will be understood that it may be applicable to many other uses for transforming the discontinuities of a surface into audible or visible intelligence. For example, it may readily be applied in measuring the roughness of a iinished machi ned surface, in which case the member 3Q would be connected to a suitable feeler of the proper contour and size which could be caused to trace the surface of a predetermined path in order that undulations and discontinuities of the surface may be imparted to the oscillatory mechanism, including the support 4D, and primarily to the movable wire or rod 44 so as to cause audible or visible manifestations indicative of the nature of the surface being traced. It will also be understood that the present invention could readily be applied to a microphone where mechanical vibrations are transferred or converted to changes of electric current. In fact, it may be applied wherever current and magnetic Variations are interrelated.

From the above description, it will be apparent that there has been provided an electronic pickup or device for converting mechanical motion to electrical energy which in turn could be converted to audible or visible signals and in which the movable mechanical means has a negligible mass so as to be very well adapted for use as a phonograph pickup. Furthermore, the device is capable of translating mechanical vibrations of a wide range of frequencies into a varying electric current for the operation of any electro-responsive device such as a loud speaker or the like. As was pointed out above, when employed in connection with a phonograph it has numerous advantages over crystal pickups or magnetic pickups heretofore employed. Furthermore, the output thereof is great in proportion to its weight so as to have a very decided advantage over magnetic pickups employed heretofore. Also by employing a permanent magnet or bar magnet such as 33 of high retentivity, the flux in the interelectrode space of the diode I0 is of high density. A slight change in such flux due to even an innitesimal movement of the wire or rod M will produce large changes in the plate current, thus ensuring a very sensitive device. In addition; the electronic pickup is admirably adapted for converting low frequency mechanical motions with a high degree of delity not present in devices used heretofore.

While there has been shown and described a particular embodiment of the present invention 1n a particular application, it will be obvious to those skilled in the art that changes and modi- .cations may be made Without departing from the present invention. It is therefore aimed in the appended claims to cover all such changes and modifications as fall Within the spirit and scope of the present invention.

What is desired to be secured by Letters Patent of the United States is:

1. A device for translating the sound track of a transcription record or the like into electrical energy comprising a diode with a substantially cylindrical envelope having electrodes arranged for establishing an electron flow transverse of the longitudinal axis of said envelope, a magnet disposed adjacent the exterior of said diode, a pair of U-shaped flux guides affixed one to each end of said magnet and each tightly embracing said diode and producing with said magnet a path of low reluctance so as to create within said diode a magnetic eld extending in the direction of the axis of said envelope, means for varying the flux density within said diode comprising a member of magnetic material disposed adjacent said diode between said flux guides, and means for moving said member in a path substantially perpendicular to a line interconnecting said flux guides in accordance with the undulations in the sound track of said record, said moving means being provided with a record engaging stylus and supporting said member, and being movably mounted with relation to said magnet.

2. A device for translating the sound track of a transcription record or the like into a variable electrical characteristic comprising a diode of elongated configuration having electrodes adapted to produce a current flowing transverse of the llongitudinal axis of said diode, a magnet disposed adjacent the exterior of said diode, a pair of U- shaped ux guides axed to the ends of said magnet and tightly surrounding said diode and rmly supporting said diode and magnet in fixed relationship and producing with said magnet a path of low reluctance so as to create within said diode a magnetic eld extending substantially parallel to the longitudinal axis of said diode, means for varying the magnetic field Within said diode comprising a member disposed adjacent said diode with its axis parallel -to the axis of said diode, and means for moving said member in a direction lperpendicular to its axis in accordance with the undulations in the sound track of said record comprising a vibrating mechanism of nonmagnetic material of minimum mass, said mechanism being pivotally mounted and supporting said member and being provided with a record engaging stylus.

3. A device for translating the sound track of a record or the like into a variable electrical quantity comprising a diode of elongated configuration having electrodes adapted to produce a current iiowing transversely of the longitudinal axis of said diode, a bar magnet disposed adjacent the exterior of said diode with its axis parallel to the axis of said diode, a pair of U-shaped members of magnetic material each having its open end fastened to said bar magnet, said U- shaped members encircling said diode, one adjacent each end thereof, lsaid U-shaped members and said magnet iproducing within said diode a magnetic field substantially parallel to the axis thereof, means for varying the magnetic field within said diode comprising a movable member of magnetic material disposed adjacent the exterior of said diode so as to extend between said U-shaped members when in one predetermined position, a non-magnetic vibrating mechanism fastened to said magnet including a pivotally mounted member fastened to said movable member, and a stylus carried by said movable member for engaging the sound track of said record.

4. A device for translating the sound track of a record or the like into a variable electrical quantity `comprising a diode of elongated configuration having electrodes adapted to produce a current flowing transversely of the longitudinal axis of said diode, a bar magnet disposed adjacent the exterior of said diode with its axis parallel to the axis of said diode, a pair of U-shaped members of magnetic material each having its open end fastened to said bar magnet, said U- shaped members encircling said diode, one adjacent each end thereof, said U-shaped members and said magnet producing within said diode a magnetic eld substantially parallel to the axis thereof, means for varying the magnetic field within said diode comprising a movable rod of magnetic material disposed adjacent the exterior of said diode so as to extend between said U- shaped members when in one predetermined position, the axis of said rod being parallel with Ithe axis of said diode, a non-magnetic vibrating mechanism fastened to said magnet including a pivotally mounted member supporting said rod, and a stylus carried by said movable member for engaging the sound track of said record.

5. A device for translating the sound track of a record or the like into a variable electrical quantity comprising a diode of elongated configuration having electrodes adapted to produce a current flowing transversely of the longitudinal axis of said diode, a bar magnet disposed adjacent the exterior of said diode with its axis parallel to the axis of said diode, a pair of U-shaped members of magnetic material each having its open end fastened to said bar magnet, said U- shaped members encircling said diode, one adjacent each end thereof, said U-shaped members and said magnet producing within said diode a magnetic field substantially parallel to the axis thereof, means for varying the magnetic field within said diode comprising an elongated movable member of magnetic material disposed adjacent the exterior of said diode so as to extend between said U-shaped members when in one predetermined position, the axis of said movable member being substantially parallel with the axis of said `bar magnet, a non-magnetic vibrating mechanism fastened to said magnet including a stylus mount having one end thereof fastened to said movable member, resilient means for pivotally supporting said stylus mount, and a stylus carried by said mount for engaging the sound track of said record.

HANS KOHN RICHARDS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number ,Name Date 749,791 Hewitt Jan. 19, 1904 1,417,912 Hewitt May 30, 1922 1,660,321 Bodde Feb. 28, 1928 1,720,824 Donle July 16, 1929 1,836,569 Benjamin --.l Dec. 15, 1931 2,118,608 Kendall May 24, 1938 2,165,307 Skellett July 11, 1939 2,437,374 Burroughs Mar. 9, 1948 FOREIGN PATENTS Number Country Date 660,398 Germany May 21, 1938 

